Automotive lighting assemblies are increasingly making use of light emitting diodes (LEDs) as light sources due to their reliability, power efficiency and minimal production levels of thermal energy as a byproduct, as compared to incandescent light sources. With improvements in LEDs, it has recently become possible to construct high output devices, such as headlamp assembly, using LEDs as the sole light source.
While LEDs do offer advantages over other light sources, such as incandescent or gas discharge sources, they also have some weaknesses. In particular, LEDs are susceptible to over-voltages, wherein too much voltage is applied to their semiconductor junctions, resulting in too much current flowing through the junctions, damaging the LED and shortening its life. Also, if too little current is supplied, LEDs produce less light (fewer lumens) and the lighting assembly may not output sufficient lumens to meets safety and/or regulatory requirements. As a further complication, the appropriate voltage/current levels for LEDs change with the temperature at which the LEDs are operated. All of these issues are further exacerbated when the LEDs are high output types, such as those which would be desired for use in headlight assembly.
As automotive electrical assembly typically experience relatively wide voltage swings and as automotive assemblies typically must operate over wide temperature ranges and conditions, to date it has been difficult to provide appropriate electrical power to LED light sources.
To date, the power supply assemblies employed for high output automotive LED assemblies have been serial supply assemblies wherein the LEDs are configured serially across the power supply, as this required the power supply to regulate a single voltage, across which all the LEDs appeared. While serial power supplies do minimize the expense of the power supplies, they suffer from the fact that, if one or more LEDs in the series circuit are damaged resulting in an open circuit, the lighting assembly will cease to illuminate. Conversely, when one or more LEDs in the circuit are damaged and short the circuit, the remaining LEDs in the circuit will be subject to an over-current and will have a decreased lifetime, at best. Also, each LED used in an assembly has its own unique operating characteristics in an assembly, due to fabrication process differences in manufacturing the LED and/or the operating conditions experienced by the LED, such as how well it is cooled in the assembly. Serial power supplies inevitably treat all of the LEDs in the serial circuit the same, thus averaging the individual LEDs' characteristics with the result that some LEDs will be overdriven and some underdriven.
It is desired to have a power supply for LED-based automotive lighting assembles, particularly high output lighting assemblies such as headlight assemblies, which is not subject to these problems.